Prosthetic devices which replace biological limbs usually interface through a hard cup-shaped shell, referred to as a socket, which encloses a residual limb. In order to transfer the necessary forces, sockets are typically fabricated with composite materials, such as carbon fiber. Compliant materials such as urethane, silicone, and/or cotton or wool fabrics typically are used between the residual limb and socket to cushion and distribute forces within the socket.
Suction is the present preferred method to affix and stabilize the socket to the residual limb. Active regulated vacuum pumps, unidirectional air valves, neoprene sleeves, and silicone suction liners with distal tension pins are among the approaches commonly used to achieve sufficient vacuum to hold sockets to residual limbs. Although usually more effective than earlier mechanical fixation techniques using belts or straps, several factors are not addressed by extant vacuum attachment approaches.
To provide proper force distribution and obtain adequate vacuum for socket stability, minimal clearance inside the socket is required. Residual limbs confined within sockets, however, often undergo changes in volume and sometimes shape as well. Non-contiguous socket shells have demonstrated greater tolerance of volume changes, but attachment remains problematic. To accommodate volume differentials experienced between the residual limb and conventional sockets, one or more fabric layers, or socks, are commonly worn between the limb and socket. Imposition of porous fabric, which does not retain vacuum, has prompted the use of either neoprene sleeves overlaid to seal the juncture between the open socket end and proximal limb, or elastomer skin-contact liners with integral distal tension pins which lock within the socket. Neoprene sleeves used to seal sockets to residual limbs quickly develop pinhole leaks as the edge of the socket is bumped into any non-compliant surface, compromising suction and thus socket stability. Socket liners with distal tension pins stretch longitudinally and thus fail to distribute distal tensile force over the entire residual limb, often creating localized tissue disruption.
Muscles remaining from amputation within a residual limb usually lose the skeletal connection necessary for their original function, so naturally atrophy. In an effort to stabilize residual limb volume, patients are furthermore routinely encouraged to avoid contraction of viable residual muscle. Compliance from the resultant fatty tissue surrounding the bone of a residual limb degrades proprioception, and often impairs prosthetic positional control. This flaccid tissue as well provides little protection for painful distal neuromas, which often form at nerve resection sites. Disuse of residual muscle compounds circulatory issues imposed by amputation, in that muscle activity in biologically intact limbs normally pumps fluids through the body. This results in intolerance to cold temperatures for many amputees, and can exacerbate phantom pain.
Extrinsic muscle stimulation, particularly if applied during contraction, has been repeatedly shown to increase both size and strength of muscle tissue. For this reason, functional muscle stimulation is commonly used to allay atrophy or improve muscle function. This use, however, has been limited to largely pre-programmed stimulation patterns in clinical settings.
A need exists whereby a prosthetic socket may be definitively secured to a residual limb during normal activities, with minimal repercussion on the residual physiology.